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Some thoughts on Rusty's improved cooling with the lower viscosity
oil. If you recall, there was an earlier discussion about drilling out the
oil passage in the iron end plate and taking the oil from there. I believe Ken
Welter did this, and obtained about a (15 degree?) improvement in oil
temperature. The only way this effect could be possible is that the pump is
working less hard, and there is therefore less heat being generated in the oil
by the action of the pump.
This could relate to Rusty's experience, namely a lower viscosity oil would
require less power input to pump, and the heating due to the pump would be less.
Add to this the beneficial effect of lower viscosity improving the heat
rejection in the heat exchanger, and I can believe the improvement is related to
viscosity change.
There is an interesting NACA paper "Cylinder-Temperature Correlation of a
single cylinder liquid cooled engine" published in 1946 where they studied the
effects of flow rate and coolant properties on cooling effectiveness. The
correlation of lots of data where they varied the ethylene glycol concentration
from 0 to 97% allowed them to determine the relative effects of heat capacity,
thermal conductivity, and viscosity of the cooling medium. They found that
high specific heat, high thermal conductivity, and low viscosity are desirable
characteristics of a good cooling fluid. In order of importance,
Thermal conducivity (k) > heat capacity (Cp) > viscosity
(v).
Rusty is not changing k or Cp with the lighter weight oil, and the
viscosity change only affects the heat transfer effect slightly, so I am left
with the conclusion that it is the decrease in absorbed work that makes the
biggest difference.
Bill Schertz
KIS Cruiser # 4045
----- Original Message ----- From: Russell Duffy [mailto:13brv3@bellsouth.net] Sent: 9/26/2004 4:39:16 PM To: flyrotary@lancaironline.net Subject: [FlyRotary] Re: Oil viscosity
5W30 seems a bit low.
>
>
I was thinking the same
thing. Leon, will an
extremely light oil be able to keep the
seperation
between the bearings and shaft? On the other hand,
will
it 'wet' the walls of the cooler better and
thereby transfer heat more
efficiently?
Since when it 30 weight
oil is "extremely light"? You might call it extremely standard, or
extremely normal :-) As I understand it, the first number is the cold
rating, which is just for cold starting, so it doesn't have any effect on normal
operation.
BTW, I made about 3
trips around the airport (on the ground of course), and just made it to 140
degrees. I'm convinced it's cooling much better with the lighter (though
not extremely light ) oil. Can't wait to fly it, but next Sunday
is the earliest possible opportunity, and probably more like the next
weekend.
Rusty (Airbike project
for sale cheap on eBay now, gotta make room in the garage)
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